Configure Spanning Tree Protocol (STP) on a Catalyst Switch that runs Catalyst OS (CatOS) Software
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Introduction
This document provides instructions on how to configure the Spanning
Tree Protocol (STP) on a Catalyst switch that runs Catalyst OS (CatOS)
Software.
Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on bridges
and switches. The specification for STP is IEEE 802.1D. Spanning tree allows a
network design to include spare (redundant) links to provide automatic backup
paths if an active link fails, without the danger of bridge loops, or the need
for manual enabling/disabling of these backup links. The main purpose of STP is
to ensure that you do not create loops when you have redundant paths in your
network. Loops are deadly to a network and can consume all available CPU
resources and bandwidth.
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Requirements
To perform the steps described in this document, you need to have this
equipment:
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A switch with Catalyst OS (CatOS) software version 6.3 or later that
is installed and powered on.
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You must have completed the initial configuration in
Configure
a Catalyst Switch with Cisco Network Assistant
document.
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A console cable. For more information about cables, refer to
Cable
Descriptions.
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Terminal emulation software such as HyperTerminal.
Note: For more information on how to create a terminal connection, refer
to Create a
HyperTerminal Connection.
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Complete the LAN Addressing Worksheet, Management VLAN Worksheet,
Secure Server VLAN Worksheet, Guest VLAN Worksheet as instructed in the
Site
Survey.
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Connect your PC to the Catalyst Switch
Follow these steps to connect your PC to the Catalyst
Switch:
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Connect a PC to the switch with a console cable.
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Create a HyperTerminal connection to your switch. For more
information, refer to
Create a
HyperTerminal Connection.
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Log into the switch with the login and password that you entered in
fields B10 and B11 of the switch Worksheet.
Username:admin
Password:
Note: If you do not know the password for your switch, refer to
Manually
Reset the Password on a Catalyst Switch.
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Type enable and press Enter to
access the privileged mode. Type the enable password that you entered in field
S5 of the Switch Port Assignment Worksheet.
Switch_CatOS>enable
Password:
Switch_CatOS><enable>
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Configure Spanning Tree Protocol
This section explains how to configure the Spanning Tree Protocol on a
Catalyst Switch.
Configure the Spanning-Tree Mode
The switch supports three spanning-tree modes: PVST+, rapid PVST+, or
MSTP. By default, the switch runs the PVST+ protocol. For more information on
modes refer to
Prepare
to Configure Spanning Tree Protocol on a Catalyst Switch.
Follow these steps to change the spanning-tree mode if you want to
enable a mode that is different from the default mode of
PVST+.
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Type set spantree mode {pvst+ | mst | rapid-pvst+ |mistp |
mistp-pvst+} and press Enter to configure a
spanning-tree mode. In this example, PVST+ is selected which enables PVST+
spanning-tree mode.
Switch_CatOS><enable>set spantree mode pvst+
Note: The default mode is rapid- pvst+
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Type show spanning-tree summary and press
Enter to verify your configuration.
Switch_CatOS><enable>show spantree
VLAN 1
Spanning tree mode PVST+
Spanning tree type ieee
Spanning tree enabled
Designated Root 00-09-e9-19-94-80
Designated Root Priority 32768
Designated Root Cost 0
Designated Root Port 1/0
Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Bridge ID MAC ADDR 00-09-e9-19-94-80
Bridge ID Priority 32768
Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Port State Role Cost Prio Type
------------------------ ------------- ---- -------- ---- ------------------
2/1 not-connected - 4 32
2/2 not-connected - 4 32
3/1 forwarding DESG 100 32 Shared, PEER(STP)
3/2 not-connected - 100 32
3/3 forwarding DESG 19 32 P2P
----Output truncated for brevity------
Configure the Root Switch
The switch maintains a separate spanning-tree instance for each active
VLAN configured on it. A bridge ID, consists of the switch priority and the
switch MAC address, is associated with each instance. For each VLAN, the switch
with the lowest bridge ID becomes the root switch for that VLAN.
Follow these steps to configure the Root switch:
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Type set spantree root vlan_id command and press
Enter in order to set the priority of the switch to 8192 for
the VLAN. The vlan_id is the Default VLAN number which you
entered in the field L7A of the LAN Addressing Worksheet.
Note: The default priority for switches is 32768. When you set the
priority with this command, you force the selection of the configured switch as
the root switch for the particular VLAN number as it has the lowest priority
value of 8192.
Switch_CatOS><enable>set spanning-tree root 20
VLAN 20 bridge priority set to 8192
VLAN 20 bridge max aging time unchanged at 20
VLAN 20 bridge hello time unchanged at 2
VLAN 20 bridge forward delay unchanged at 15
Switch is now the root switch for active VLAN 20
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Type set spantree root vlan_id command and press
Enter in order to set the priority of the switch to 8192 for
the VLAN. The vlan_id is the Network Management VLAN number
which you entered in the field L7B of the Network Management VLAN
Worksheet.
Switch_CatOS><enable>set spanning-tree root 21
VLAN 21 bridge priority set to 8192
VLAN 21 bridge max aging time unchanged at 20
VLAN 21 bridge hello time unchanged at 2
VLAN 21 bridge forward delay unchanged at 15
Switch is now the root switch for active VLAN 21
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Type set spantree root vlan_id command and press
Enter in order to set the priority of the switch to 8192 for
the VLAN. The vlan_id is the Secure Server VLAN number which
you entered in the field L7C of the Secure Server VLAN Worksheet.
Switch_CatOS><enable>set spanning-tree root 22
VLAN 22 bridge priority set to 8192
VLAN 22 bridge max aging time unchanged at 20
VLAN 22 bridge hello time unchanged at 2
VLAN 22 bridge forward delay unchanged at 15
Switch is now the root switch for active VLAN 22
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Type set spantree root vlan_id command and press
Enter in order to set the priority of the switch to 8192 for
the VLAN. The vlan_id is the Guest VLAN number which you
entered in the field L7D of the Guest VLAN Worksheet.
Switch_CatOS><enable>set spanning-tree root 23
VLAN 23 bridge priority set to 8192
VLAN 23 bridge max aging time unchanged at 20
VLAN 23 bridge hello time unchanged at 2
VLAN 23 bridge forward delay unchanged at 15
Switch is now the root switch for active VLAN 23
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If you are not certain that all the switches in your network have
a priority that is greater than 1 then set the priority of your desired root
bridge to 1. Type set spantree priority 1 vlan_id. In this
example the vlan_id is the Default VLAN number which you
entered in the field L7A of the LAN Addressing Worksheet.
Switch_CatOS><enable>set spantree priority 1 20
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Issue the show spantree vlan_id command in order
to verify that this Switch is the root of all the appropriate VLANs.
From the output of this command, compare the MAC address of the
switch that is the root switch to the MAC address of the switch from which you
issued the command. If the addresses match, the switch that you are in is the
root switch of the VLAN. A root port that is 1/0 also indicates that you are at
the root switch. This is the sample command output:
Switch_CatOS><enable>show spantree 20
VLAN 20
spanning-tree enabled
spanning-tree type ieee
Designated Root 00-10-0d-b1-78-00
!--- This is the MAC address of the root switch for VLAN 1.
Designated Root Priority 1
Designated Root Cost 0
Designated Root Port 1/0
Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Bridge ID MAC ADDR 00-10-0d-b1-78-00
Bridge ID Priority 1
Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
This output shows that this Switch is the designated root on the spanning tree
for VLAN 20. The MAC address of the designated root switch, 00-10-0d-b1-78-00,
is the same as the bridge ID MAC address of Switch, 00-10-0d-b1-78-00.
Another indicator that this switch is the designated root is that the designated
root port is 1/0.
Configuring a Secondary Root Switch
When you configure a Catalyst switch as the secondary root, the switch
priority is modified from the default value (16384) to 28672. The switch is
then likely to become the root switch for the specified VLAN if the primary
root switch fails. This makes us assume that the other network switches use the
default switch priority of 32768 and therefore are unlikely to become the root
switch.
You can execute this command on more than one switch to configure
multiple backup root switches. Use the same network diameter and hello-time
values that you used when you configured the primary root switch with the
spanning-tree vlan vlan-id root primary global configuration
command.
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Log in to the privilege mode of the alternate switch which you want
it to become secondary root switch for a specific VLAN.
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Type set spantree root secondary 20 and press
Enter.
Switch_CatOS><enable>set spantree root secondary 20
VLAN 20 bridge priority set to 16384
VLAN 20 bridge max aging time unchanged at 20
VLAN 20 bridge hello time unchanged at 2
VLAN 20 bridge forward delay unchanged at 15.
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Type set spantree root secondary 21 and press
Enter.
Switch_CatOS><enable>set spantree root secondary 21
VLAN 21 bridge priority set to 16384
VLAN 21 bridge max aging time unchanged at 20
VLAN 21 bridge hello time unchanged at 2
VLAN 21 bridge forward delay unchanged at 15.
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Type set spantree root secondary 22 and press
Enter.
Switch_CatOS><enable>set spantree root secondary 22
VLAN 22 bridge priority set to 16384
VLAN 22 bridge max aging time unchanged at 20
VLAN 22 bridge hello time unchanged at 2
VLAN 22 bridge forward delay unchanged at 15.
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Type set spantree root secondary 23 and press
Enter.
Switch_CatOS><enable>set spantree root secondary 23
VLAN 23 bridge priority set to 16384
VLAN 23 bridge max aging time unchanged at 20
VLAN 23 bridge hello time unchanged at 2
VLAN 23 bridge forward delay unchanged at 15.
Configure Port Fast
Spanning-tree Port Fast causes a spanning-tree port to enter the
forwarding state immediately, bypassing the listening and learning states. The
Port Fast feature must be used only when connecting a single end station or a
server to an access or trunk port.
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To enable Port Fast on a switch port enter the command set spantree
portfast module_num/port_num enable, where module_num and
port_num must be replaced with the actual module number and
port number in digits.
switch_CatOS><enable>set spantree portfast 1/1 enable
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To disable Port Fast on a switch port enter the command set
spantree portfast module_num/port_num disable, where
module_num and port_num must be replaced with
the actual module number and port number in digits.
switch_CatOS><enable>set spantree portfast 1/1 disable
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Disabling Spanning Tree
STP is enabled by default on VLAN 1 and on all newly created VLANs up
to the spanning-tree limit. Disable STP only if you are sure there are no loops
in the network topology.
When STP is disabled and loops are present in the topology, excessive
traffic and indefinite packet duplication can drastically reduce network
performance.
Follow these steps to disable STP:
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Type set spantree disable vlan-id and press
Enter to Disable STP on a per-VLAN basis.
Switch_CatOS><enable>set spantree disable 23
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Type clear spantree root 1 vlan-id and press
Enter to reset the spanning-tree bridge priority value to the
default setting.
Switch_CatOS><enable>clear spantree root 23
VLAN 23 bridge priority set to 32768.
VLAN 23 bridge max aging time set to 20.
VLAN 23 bridge hello time set to 2.
VLAN 23 bridge forward delay set to 15.
Note: The switch priority for the VLAN 23 changes to the default value
of 32768.
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Next Step
You have completed this procedure.
To make further changes to your switch, refer to the
Switch
Support Page.
To configure other devices in your network, refer to the
Configuration
Overview Page.
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Troubleshoot the Procedure
This section provides information about common problems that you may
encounter. If this information does not solve your problem, contact the
SMB
Technical Assistance Center (SMB TAC) for assistance.
Problem
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Cause(s) and Suggested Solution(s)
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The switch boots and displays several error messages but does
not display a prompt in HyperTerminal.
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Press Enter to clear the output and display
the switch prompt.
For further assistance, contact the
SMB
Technical Assistance Center (SMB TAC).
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